The measurement of metabolism or metabolic rate has application in a varied number of fields including exercise physiology, biology, physiology, biochemistry, nutrition, fitness, cardiology, pulmonology, endocrinology and physical therapy. Such measurements find utility in physiological measures, e.g. as part of routine physical examinations, as adjuncts to medical treatments and monitoring functions relating to a variety of conditions related to the heart, respiratory system, obesity and others. Such measurements are also widely used in conjunction with sports and general fitness programs.
The most common method of carrying out such determinations is by indirect calorimetry. This is a non-invasive means of determining the body's metabolic activity through oxygen consumption and carbon dioxide production. The method, fundamentally, entails measurement of ventilation, sampling the subject's inhalation and exhalation and determining the oxygen and carbon dioxide content of each. These measurements can be used to quantify the body's energy expenditure and metabolic state, and may also be utilized to detect certain disease processes such as, for example, obstructive lung disease, heart disease and peripheral vascular disease.
There are a number of systems available to carry out the determination of the oxygen/carbon dioxide in exhaled air. These vary in complexity, principle of operation and design. The most basic of these is the Douglas bag, which has been in use for about fifty years. This is a static system which consists of a large, flexible bag having a capacity typically from about fifty to as much as two hundred liters. The Douglas bag can be used as a rebreathing apparatus that is well suited for such measurements as lung profusion. In this measurement, the subject continuously inhales a mixture of air and a radioactive gas from the bag so that lung morphology and functioning may be studied with specialized equipment by monitoring the movement of the radioactive gas in the lungs. However, a static system, such as the Douglas bag, is not well suited for the collection of exhaled air, as for continuous indirect calorimetry, partially because of its volume and partially due to the length of time required, usually from about thirty seconds to one minute, for the collection of a sufficient volume of expired air. This is true even with more modem sensors and other similar improvements to the basic Douglas bag apparatus.
There are a number of systems available that have the capacity to measure and compute data from samples taken of exhaled air at given intervals of time, e.g. 15 seconds, 30 seconds, 60 seconds, or each breath. For the breath-by-breath systems, the oxygen and carbon dioxide content of samples of the exhaled gas stream is determined and compared to the inhaled gas mixture to establish a curve. These systems are potentially flawed, however, because during exercise when the frequency of breathing increases significantly, the breath-average curves developed from these measurements can become distorted with resultant loss of accuracy. The time-averaged systems suffer the disadvantage of having the results skewed due to the associated dead air space in the system consisting of tubing directing air to a constant volume chamber where it is mixed. This dead air space, in combination with the typical size of the fixed volume mixing chamber, typically one to five liters, act to decrease the overall sensitivity and accuracy of the system.
Another factor that is often discounted, but which can also distort results of indirect calorimetry, even on sophisticated equipment, is the anatomical dead space (ADS) in the body which results in alveolar air being present during inhalation, and atmospheric air during exhalation. While it is not difficult to determine the ADS volume, it has largely been ignored due to the misconception that its effect is negligible and/or that it is adequately counteracted by or compensated for by the atmospheric air in the ADS during exhalation. We have found, however, that the effect of the ADS does not appear to be self-correcting and results in appreciable errors in the determination of the volume of exhaled oxygen and carbon dioxide.
In addition to the aforementioned factors that contribute to inaccuracies in determinations of indirect calorimetry utilizing known systems, the configuration of systems in use may also contribute to variations in the results obtained. The size and efficiency of the mixing chamber, its proximity to the mouthpiece, the locus of the sampling device and the construction of the mouthpiece itself are all factors that may contribute to inaccuracies in indirect calorimetry measures. It will be appreciated that the impact of any one of these factors on the accuracy of the results will in large part be dependent on the purpose of the determination. Further, where more than one of these factors is at work in a given system, the overall detrimental effect will be even more pronounced. There are situations where accuracy is critical to determinations, such as the calculation of the dosage of certain medications, where the margin for error must be as small as possible. It will be appreciated that, the more critical the need for accuracy, the greater the impact of these factors. In accordance with the present invention, there is provided a system for indirect calorimetry that minimizes or eliminates the disadvantages of known systems as described above.